1. Field of the Invention
The application relates to a method used in a wireless communication system and related communication device, and more particularly, to a method for load balancing in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as to user equipments (UEs).
LTE technology supports packet based services only. However 3GPP does specify fallback for circuit switched (CS) services as well. To achieve this, LTE architecture and network nodes require additional functionality. CS Fallback defines a mechanism for using a CS legacy network (e.g. Global System for Mobile communications (GSM) or Universal Mobile Telecommunications System (UMTS)) to provide voice services along side of an LTE network. CS fallback provides voice and traditional CS-domain services (e.g. voice call, video call, short message service (SMS), Unstructured Supplementary Service Data (USSD), supplementary service). To provide these CS services, LTE reuses CS infrastructure when the UE is served by E-UTRAN.
A UE with multi-modes supports multiple radio access technologies (RATs), such as GSM, UMTS, and LTE systems. A connection established in LTE can be handover to GSM or UMTS system and vice versa. Since the LTE system supports packet based services only, the aforementioned CS services in the GSM or UMTS systems are not supported in the LTE system. To make the CS services available to UEs camped on the LTE network, a CS fallback procedure is designed. When the UE initiates a CS service or receives a paging of the CS services in the LTE system, the UE is forced to connect to the GSM or UMTS system using “inter-RAT handover to GSM or UMTS”, “cell change order (CCO) to GSM or UMTS”, or “RRC connection release with redirection to GSM or UTMS”. The “inter-RAT handover”, “CCO” and “RRC connection release” procedures are specified in 3GPP TS 36.331 v9.1.0.
The UE in radio resource control (RRC) connected mode performs PS data transmission and reception (e.g. ftp) in LTE network. The UE initiates a CS call or a Mobile Originated Location Request (MO-LR) by using CS fallback procedure. The network sends a RRC connection release message or a Mobility from EUTRA message to redirect the UE to UTRAN. When the UE camps on UTRAN, the UE initiates a RRC connection procedure. The UE performs random access and transmits a RRC Connection Request message including establishment cause “Originating Background Call”, triggered by PS data transmission (e.g. ftp) to the UTRAN. However, the UTRAN is designed to redirect UEs supporting LTE to the network for PS services for load balancing so the UTRAN redirects the UE to the LTE network. Therefore the UE fails to initiate the CS call. If the UE tries again, the UE is redirected back and forth between the UTRAN and LTE network.